Bug fix: attempt to avoid hangs when creating blockstores on large SSDs.

At least I think the problem is that we are doing inadvertent TRIM
operations on large (480GB) SSDs. For one, when creating an ext4
filesystem on such a blockstore, we specify "nodiscard". I toyed
with the idea of turning off "issue_discards" for the lvremove
operations when a blockstore is destroyed, but that led to old
metadata being seen when the blockstore was re-created. That led
to the last change, which was to force metadata zeroing when we
do an lvcreate of a blockstore.